1. Field of the Disclosure
This disclosure relates generally to the field of electrical resistivity well logging systems, tools and methods.
2. Description of the Related Art
Wellbores or boreholes for producing hydrocarbons (such as oil and gas) are drilled using a drill string that includes a tubing made up of jointed tubulars or a continuous coiled tubing that has a drilling assembly, also referred to as the bottom hole assembly (BHA), attached to its bottom end. The BHA typically includes a number of sensors, formation evaluation tools, and directional drilling tools. A drill bit attached to the BHA is rotated with a drilling motor in the BHA and/or by rotating the drill string to drill the wellbore. An electromagnetic wave propagation logging tool for determining electrical properties of the formations surrounding the borehole is often deployed in the BHA. Such tools are generally referred to in the oil and gas industry as the resistivity tools. These logging tools make measurements of apparent resistivity (or conductivity) of the formation that, properly interpreted, provide information about the petrophysical properties of the formation surrounding the borehole and fluids contained therein. Resistivity logging tools also are commonly used for logging wells after the wells have been drilled. Such tools are typically conveyed into the wells by wireline. The tools that use wireline are generally referred to as the wireline resistivity tools, while the logging tools used during drilling of the wellbore are generally referred to a the logging-while-drilling (LWD) or measurement-while-drilling (MWD) tools. These resistivity logging tools also are referred to as induction logging tools. For the purpose of this disclosure, the term resistivity tool or induction logging tool is meant to include all such and other versions of the resistivity tools.
A typical resistivity tool includes one or more receiver coils or antennas spaced from each other and one or more transmitter coils or antennas. Alternating current is passed through the transmitter coil, which induces alternating electromagnetic fields in the earth formation surrounding the wellbore. Voltages are induced in the receiver coils as a result of electromagnetic induction phenomena related to the alternating electromagnetic fields in the formation.
LWD resistivity tools, for the most part, make omni-directional measurements. The portion of the formation that affects the signals typically takes the shape of a torus. The antenna configuration used in these tools usually includes a number of axial slots (along a longitudinal axis of the tool and the wellbore) made in the tool body. An antenna wire loop is made by placing a wire (electrical conductor) over the slots, perpendicular to the tool longitudinal axis. The longitudinal tool axis is also referred to as the “tool axis.” A ferrite material is often placed in the slots below the wire to increase the sensitivity of such antennas. Resistivity tools also have been developed that are sensitive to the azimuthal direction of a resistivity contrast within the depth of investigation of the tool. In such tools, the antenna wire is not perpendicular to the tool axis. Therefore, the slots for the placement of the ferrite material for such antennas are also not oriented along the tool axis. Such slots are tilted relative to the tool axis and in an extreme case are formed perpendicular to the tool axis, i.e., along the radial direction of the tool body or a housing. These slots are in the form of continuous notches made in the metallic housing. However, continuous slots made into a section of an LWD resistivity tool body, such as in a section of a drill collar, reduce mechanical strength of the tool body, which can result in developing cracks when the tool body is subjected to high bending loads during drilling of a wellbore.
The disclosure herein provides improved tools, system and methods for estimating or determining an electrical property downhole.